Added separate EXTI driver for the STM32Cube side and reworked STM32L0xx board side to match. Probably still have some detail work left to do, have not tried to compile yet. The CC2500 driver for that side also needs attention. Once all of that builds again, this branch will probably be ready for preliminary review and test.

[#18] Cleaning up CC2500 SPI bus handling. It turns out that no adapter is needed here since it is all just API calls. Next will need to rebuild EXTI into a service on the STM32Cube side and create a CC2500 adapter for it, then this should be ready for review and test.